fix(markdown/table): use Token.attr[Get|Join] insteed the hardcode tabindex attr

[TheNorthMemory]

markdown-it-attrs supported extra attributes for eg: {tabindex=1} after table block, fix it.

Description

vitepress/src/node/markdown/markdown.ts

Line 235 in 8f31a4c

return '<table tabindex="0">\n'

Additional Context

refer: https://markdown-it.github.io/markdown-it/#Token.attrGet and https://markdown-it.github.io/markdown-it/#Token.attrJoin

[TheNorthMemory]

Hi there, this site https://wechatpay.im, is strictly rely on this function, because there were more than 1000+ markdown tables rended. Hopes this fix shall be merged.